Semiconductor waveguides, for example lasers or laser diodes are well known in the art with many applications in data communications, telecommunications, metrology and sensing. The most common method in achieving single frequency operation in a semiconductor laser is by etching a grating layer into the semiconductor and burying that grating layer using epitaxial overgrowth. As the grating pitch is of the order of the wavelength/refractive index, either holographic or electron beam techniques are required for their definition. In addition in many cases the added complexity of material overgrowth makes those common techniques expensive.
One of the simpler and more reliable laser devices available is the self aligned ridge laser. Such a device is described in U.S. Pat. No. 5,059,552.
FIG. 1 illustrates a typical example of a “ridge” laser or lasing device 7. The representation of a finished ridge laser device is a simplified version for the purposes of explanation. In operation, light 9 is primarily emitted from shaded region 8. The formation of a “ridge” 6 is effected by etching into a layered material, as shown in FIG. 2, comprising a series of at least four epitaxial layers 1, 2, 3, 4 formed on a semiconductor substrate 5. For a n-type substrate 5, the top layer 1 which is the contact layer comprises p-type material, the second layer 2 is a cladding layer and also comprises p-type material. The third layer 3 typically comprises a number of undoped active layers which are used for light guiding and gain purposes and may be composed of bulk, quantum well or quantum dots. The fourth layer 4 is an n-type cladding layer. It will also be understood that suitable etch stopping layers may also be incorporated into the structure 20. As with other semiconductor devices, typically a large number of ridge lasers are formed on a single semiconductor wafer and subsequently divided. Accordingly, the structures illustrated should be taken as only a part of a larger semiconductor body.
For the case of a InP laser emitting in the 1.2–2.0 μm wavelength range, the epitaxial top layer 1 is typically InGaAs, with the second and fourth layers and the substrate typically InP, with layer 3 typically containing InGaAsP and/or InGaAlAs. Other material combinations are also possible. It will also be appreciated that alternative semiconductor materials, e.g. those based on GaAs, GaSb, or GaN would incorporate different epitaxial layers and could require alternative etchants.
The process of manufacturing a “ridge” laser, as illustrated in simplified form in FIG. 3, commences with the formation (30) of the outline of the ridge in a layer of resist material 40, as shown in FIG. 4, on the top layer 1, using a suitable lithographic technique. The shape of the resist material is determined by the mask used in the lithographic process. The subsequent outline of the ridge formed will be determined by the outline of the mask.
The next step in the process is to etch 31 the structure to remove the top layer and part of the second layer in regions not covered by the resist material. The thickness (t) of the remaining portion of layer 2 in the region which has been etched 51, 52 contributes to the characteristics of the finished “ridge” laser. In regions covered by the resist material, the top layer and second layer are substantially unaffected by the etching process, thus leaving a raised surface or “ridge” effectively matching the mask outline. The width of the ridge (w) formed matches that of the outline formed by the resist material 40.
Single spatial mode output is essential for many applications and is typically obtained by an appropriate choice of ridge width (w) and etch depth, or more correctly the remaining thickness (t). Typically, imposing a ridge width (w) in the range of 2–4 μm while t is of the order of 0.1–0.5 μm. Achieving this single mode is an objective of the ‘ridge’ and is relatively easily obtainable using conventional techniques.
The third step 32 in the process is to apply a dielectric coating 60 over the structure, as shown in FIG. 6a. The next step 33 involves removing a portion 61 of the dielectric material 60 covering the top surface of the ridge, as illustrated in FIG. 6b, using a conventional etching technique. The final step 34 is to apply a metal contact layer 62 on the portion of the ridge 6 not covered by the dielectric material to form a metal contact 62, as shown in FIG. 6c. Other steps not shown include an alloy and thinning of the substrate to approximately 100–120 μm, a further metal coating step to apply a metal contact layer to the substrate with subsequent alloy, a cleaving step in which the ridge laser is cleaved at a particular point to define its end, and the breaking up of the wafer into individual ridge laser devices.
Although such “ridge” laser devices are reliable, there is a tendency of such structures to operate with multi-longitudinal modes. The work of DiChiaro [L. DeChiaro, J. Lightwave Technology, Vol 8 Nov. 1990 pp 1659–1669, J. Lightwave Technology, Vol 9 Aug. 1991 pp 975–986] showed that the introduction of a defect at a fractional position of the cavity length could convert multi-longitudinal modes into a single longitudinal mode. This method is however rather crude and introduces damage into the laser. Further to this, Patterson et al and Kozlowski et al [B. D. Patterson et al, Microelectronic Engineering, Vol 27 1995, pp 347–350, D. A. Kozlowski et al, Electon. Letters, Vol 31, April 1995, pp 648–650] used focused ion beam etching to create a series of holes along the length of a laser cavity. Kozlowski monitored the spectrum of the laser during this process, and showed that the etching allowed for an improved spectral performance (Side Mode Suppression Ratio—SMSR) through the enhancement of the effect by several ‘defects’ acting together. However, this work was on fully processed lasers, i.e. post laser fabrication, and has limited commercial application. Further work by the present inventor [B. Corbeft and D. McDonald, Electron Letters, Vol 31, December 1995, pp 2181–2182]showed that the integration of the defect could be incorporated into the standard process sequence for ridge waveguide lasers.
The integration of a suitable defect may be incorporated using electron beam lithography by the placement of a gap in the mask forming the ridge, thus resulting in an omission of a section of resist material. Accordingly, the etching process forms a slot 70 between two sections 71, 72 of the ridge as shown in FIG. 7. More than one slot may be required to achieve a desired characteristic in each ridge laser device. An implementation, as published by the present inventor [supra], having a slot width “s” of 0.5–1.0 μm allows for accurate reproduction using electron beam lithography. The width of the slot “s” is determined by the gap width in the mask forming the ridge. The etch depth (d) of the slot is the same for both the spatial mode and the longitudinal mode, resulting from the processing of both ridge and slot in the same process step, i.e. the depth (d) of the slot 70 equals the height (h) of the ridge. It will be noted that the slot formed is dimensionally along the longitudinal axis of the ridge.
Electron beam lithography is a ultra-high definition direct-write lithographic process where is accomplished by omitting to write (or in some processes writing) a pattern of electrons onto a resist layer.
Conventionally, the length of the ridge and accordingly the length of the laser are defined by marking the edge of the processed material with a diamond scribe perpendicular to the ridge. Increasing sophistication of tools now permits an absolute marking accuracy of a few microns with relative marking accuracy less than 1 μm. However, in order to have accurate absolute markings with respect to the slot location, a special cleave feature is defined lithographically. This is achieved using a non-selective dry etch through the active layers and a special crystallographic wet etch that ends in a sharp line; this being the intersection between two crystal planes, as shown in FIG. 7. The edge then serves as the location of the cleave plane and hence the end of the laser (referred to as the facet). For example, in FIG. 7 the ridge laser device would have its end defined by the line a—a after the cleaving process. The appropriate wet etchant for InP substrate is HCl. Again, it will be appreciated that the etchants described hereinbefore are specific to an InP substrate and that alternative substrates such as GaAs may require different etchant materials. The registration of the slot with respect to the cavity length is obtained by the high resolution of the direct write electron beam lithography system. The cleave feature is formed in a separate process to the formation of the ridge and slots.
As detailed above, this existing technique is implemented using direct write electron beam lithography, which is highly accurate in its definition of features and in referencing between lithographic levels. However, use of this technique is slow, expensive and does not deliver sufficient cost benefit to be favoured over alternative techniques for single frequency lasers such as Distributed Feedback (DFB) devices. Due to the large costs associated with purchasing and running direct electron beam lithographic equipment, it will be appreciated that it is not commercially feasible to use such equipment for production purposes. To be commercially viable the process needs to be implemented with more cost efficient techniques such as optical lithography. In the present application, the use of the word optical is intended to include any lithographical process using the projection of a resist modifying flux through suitable masking apertures, and includes the use of visible list, deep UV, or scattering electron beam lithography. The use of optical lithography has, however, associated shortcomings such as resolution and alignment accuracy. In particular, to the resolution requirement to define, for example, a ridge of width of the order of 3 μm, having a slot with a gap of the order of 0.5 μm, is not obtainable using conventional optical lithographic techniques.
Using optical lithography to produce a ridge outline having a gap in the resist layer is ineffective. The resist pattern 82 becomes rounded in regions corresponding to the corner regions of the mask 80 and leads to an ill-defined resist surface pattern compared to the results 81 achieved using direct write electron beam lithography, as shown in FIG. 8. This rounding of corners in resist patterns as reproduced on a ridge subsequently by etching, results in a general degradation in the single frequency performance of the resulting “ridge” laser. If poor contact is made between the mask and the imaging resist during pattern transfer the resultant degradation in corners can be more severe due to diffraction, which results in further degradation in the single frequency performance of the laser device.
It is important for InP lasers that the ridges are aligned parallel to the major flat (crystal planes) of the wafer. It will be understood that all wafers have the crystal axes identified by the roundness being flattened, which is what is meant by the term crystal flat.
Accordingly, it would be beneficial if a “ridge” laser could be designed having single frequency performance similar to that available using direct write electron beam lithography but which could be manufactured using conventional optical lithography techniques.
A further important alignment is in the referencing between a topographic feature for example a slot in a waveguide, and the laser facet. The laser facet is the break in the laser which is along a crystal axis and which is the mirror providing feedback into the laser. The position of the laser facet is defined by the cleave feature as shown in FIG. 7. The reflectivity of the facet may be changed by the application of coatings. It is advantageous to have this referencing as accurate as possible for best reproducibility in device performance. The cleave feature is a notch formed by etching. FIG. 7 illustrates that the ridge is the only structure extending above the etched surface and that the cleave feature starts at the etched level. It will be appreciated that conventionally, whilst areas adjacent to the ridge may be etched, other regions of the semiconductor structure may be of similar height to the ridge. In these situations the cleave feature will extend from the top layer through to the substrate. In the direct write electron beam process prior art the cleave feature is formed in a different step of the process to the ridge and slot. Alignment of the slot and cleave feature is achieved by the resolution available through electron beam lithography.
It would further be beneficial if conventional optical lithography manufacturing processes could be modified to allow in manufacture registration or alignment of slots or other topographic features to the cleaving feature and hence facet of a opto-electronic device, for example a laser.
Accordingly, there is a need for an improved semiconductor device, e.g. ridge laser and method of making same.